The 4-year-old son of Malibu Moon was otherwise in good physical shape, Ellis said, and was shipped back to the trainer's Santa Anita base on Monday.

"We're going to do some bloodwork (Tuesday) and a throat culture and find out exactly what's going on," Ellis said. "But it doesn't appear to be anything serious."

Declan's Moon, who earned the Eclipse Award as the nation's top juvenile in 2004 after an undefeated record of four wins, including three graded stakes, spent more than 16 months on the shelf after undergoing surgery to remove a bone chip from his left knee in March, 2005.

He returned to action in mid-July and finished second in an optional claiming allowance event at Hollywood Park.

Maryland-bred Declan's Moon was made eligible for the Breeders' Cup as a horse of racing age after owner Samantha Siegel made a $150,000 payment to the program last week, but Ellis remained non-committal to any future plans for Declan's Moon.

"We'll just regroup now," Ellis said. "But in the grand scheme of things, this is probably the best possible thing (to explain the gelding's poor performance in the Pat O'Brien)."

From seven career starts, Declan's Moon owns five wins and a second for earnings of $638,300.